1. Field of the Invention
The present invention generally relates to fiber optic cables, and more particularly, relates to a breathable fiber optic cable which is ventilated to reduce hydrogen partial pressure next to the fiber.
2. Description of the Related Art
Fiber optic cables used for oil well applications where the cable is installed in the well have been problematic. One of the key concerns has been hydrogen darkening of the fiber. In the downhole environment, there is often free hydrogen present. Hydrogen can easily permeate the materials of the cable and diffuse into the optical fiber. The presence of hydrogen in the fiber will create an increase in transmission loss. This can make the systems that use the fiber—fiber optic pressure sensors and distributed temperature sensing—difficult if not impossible depending on the optical loss level in the fiber. Due to these issues, the cable structures used will often use hydrogen getters of various sorts to consume the present hydrogen. Other designs that resist hydrogen will include the use of a carbon layer on the optical fiber which will slow the diffusion of hydrogen into the glass. Unfortunately, some of the hydrogen getters are not suitable for temperatures above 150° C. and at higher temperatures are suspected of actually releasing the hydrogen they have captured. The use of a carbon layer has shown to lose its effectiveness as temperatures rise. Another feature used to improve the survivability in regard to hydrogen is to use additional metal layers in the cable that slow the diffusion of hydrogen. Metals such as copper, gold or aluminum are particularly good in slowing hydrogen diffusion. Manufacturers like FiberGuide and Oxford Electronics offer metal coated fibers. Unfortunately, although these options extend the life of the cable, these cable structures will show increased optical loss over time in the presence of hydrogen.
For example, one fiber optic cable uses a pure silica core fiber in cable structures designed for temperatures around 150° C. The purpose of this type of fiber is that the hydrogen, although it will still diffuse into the core of the fiber, will not react with dopants as the pure silica fiber does not have any in its core. Dopants such as germanium, fluoride and phosphorous are commonly used by fiber manufacturers to improve various attributes of the fiber. This is discussed in detail in “Development of Fibers Optic Cables for Permanent Geothermal Wellbore Deployment”, R. Normann, J Weiss and J. Krumhansl. The issue with this logic is that diffusion rates of hydrogen in a pure silica fiber or a doped fiber is the same, so in the presence of the same hydrogen concentration both fiber types will exhibit attenuation loss. Although, the pure silica core will perform better as the loss associated with reactions with dopants is greater than the loss associated with hydrogen dispersed in the interstices of the glass core of the optical fiber. The loss associated with reactions to dopants has been documented at 100° C. and is documented in a white paper written by Joshua Jacobs titled “The Impact of Hydrogen on Optical Fibers”.
Thus, the problem of survivability of fiber optic downhole cables has been approached with the intent of extending the life of the cable by creating barriers for the hydrogen. Inevitably, however, the hydrogen will get to the core of the fiber, especially at elevated temperatures.